This invention relates to an electrical power-assisted steering system for a vehicle of the kind comprising an electric motor connected through a gearbox to act on a steering mechanism of the vehicle.
For example, the gearbox may provide a connection between the motor and the steering column shaft, or directly onto a portion of a rack and pinion mechanism forming part of the steering mechanism.
The electric motor is used to assist a driver in applying torque to the steering mechanism, by applying assistance torque of the same sense, to make it easier to turn the steering wheel, for example during parking manoeuvres. Thus, operation of the motor may assist in rotating the steering column shaft, or in moving a portion of the steering rack mechanism.
The motor, which may be a multi-phase brushless star-connected permanent motor, is controlled by motor controlled means, comprising control and drive circuits, which is operative to supply current from a power supply to the motor phase windings. The phase windings of the motor are connected to the star point. Each phase is connected to a positive terminal of a power supply by a top transistor, and to a negative terminal by a bottom transistor, the two transistors defining an arm of a multiple arm bridge. This bridge forms the drive circuits, while the control circuits are provided by a microprocessor or digital signal processor or analogue signal processing or some combination thereof. The microprocessor is operative in response to signals in the torque sensor provided on the steering column to measure the torque applied by the driver, from a motor rotor position sensor providing information about motor speed and direction and optionally from signals corresponding to current flowing in the motor bridge or power supply. This information can be used in combination with the torque sensor signal and/or column position sensor signal to determine which phase windings should be energized and when. The microprocessor produces control signals which energise the transistors of the drive circuits to cause current to flow in a desired motor phase. In short the electrical power-assisted steering system provides a significant level of servo support for the driver""s steering demand.
The problem with such an electrical power-assisted steering system is that should a fault develop then the sudden loss of assist torque can be uncomfortable for the driver.
The electrically power-assisted steering system disclosed in U.S. Pat. No. 5,663,713 incorporates an unstable circuit, such as first and second cascade connected integrators having an input connected to a controller and an output connected to a detector, which signals an error when the output signal of the unstable circuit is outside an acceptable range. During normal operation, the controller supplies a signal to the unstable circuit such that its output signal remains within the acceptable range. The error signal may be used to disable the actuator, for example so as to remove power assistance from the steering system.
In the electrical power-assisted steering system disclosed in International application number PCT/GB97/00528 (Ser. No. WO97/32220) a circuit is provided for testing a drive stage of a motor. A power supply circuit comprises a contact for supplying normal drive stage current and a resistor for supplying reduced drive stage current before the normal current is supplied. A motoring circuit measures a drive stage electrical parameter such as a supply voltage and a comparator compares this with an acceptable value. If the measured parameter corresponds to a current through the drive stage which is different from an expected value, a fault is signaled and the contact is prevented from closing.
In the electrical power-assisted steering system of International patent application number PCT/GB97/02446 (Ser. No. WO98/01971) switching means such as a relay is provided in at least oral phase of the motor with the switching means being movable between a closed position in which current is able to flow in the phase winding and an open position which prevents current flowing in the respective phase in the event of a fault.
According to our invention in an electrical power-assisted steering system for a vehicle comprising an electric motor connected through a gearbox to act on a steering mechanism of the vehicle, the electric motor has a plurality of phase windings connected to a star point and a switch provided in at least one phase of the motor, the switch being movable between a closed position in which current is able to flow in the phase windings, and an open position which prevents current flowing in a respective phase winding, the system incorporating a safety circuit operative when a fault develops to smooth out the transition between a normal operation state and a shut-down state having presence and absence of servo support in assisting and not assisting said steering mechanism respectively by providing a prescribed period of damped operation as part of a shut-down phase.
A damped operation is provided by new use of existing features when combined to apply a torque to the system in accordance with the driver""s input.
The damping torque is applied to the steering system only for a relatively short period of time.
The damped state comprises an extra operating state operative between normal operation and shut-down state.
In the damped state, there may be provided a closed path for current which passes through the motor windings.
In normal operation, power is connected to a drive stage of the motor, a top set and a bottom set of transistors are provided which control the motor according to an overall assistance control objective and the switch comprises a star-point relay, the contacts of which are closed.
In the damped state the motor drive stage power is disconnected, one set of transistors are turned off, the other set of transistors are turned on, and the star point relay contacts are closed.
Thus, in the damped state, when the motor moves a back-emf may be generated in the windings. The magnitude of the back-emf may be proportional to the speed of the motor. The closed path may pass through the motor windings, the motor relay contacts, and the drive stage transistors such that, in the damped state, a current is allowed to circulate. The current may attempt to oppose the movement of the motor, as inherit in permanent magnet motors (brush or brushless).
In the shut-down state the motor drive stage power supply is disconnected, both sets of transistors are turned off, and the motor star point relay contacts are opened. In this shut-down state no current can flow through the motor windings and so no assistance torque can be developed.
The control between the states of normal operation, damped operation, and shut-down can be achieved by any convenient means. For example the control can be achieved purely as a software function, or a combined software and hardware function. The particular control chosen would depend upon the particular system design.
One possible implementation is to use a hardware circuit to invoke the damped state for a particular duration. This circuit would take over control of the drive stage, turn off one top or bottom set of transistors, turn on the other set of transistors, and hold the motor relay contacts closed. After the required period has elapsed, say between 0.25 and 2.5 seconds, the circuit would turn all of the components off, thus evoking the shut-down state. It will be noted, therefore, that the damping function is operative for a short period only.
The hardware circuit could be triggered by a signal from the main microcontroller, or from a signal from a monitoring circuit.
The system design must ensure the correct operation of the damping circuit hardware and allow the damping circuit hardware to be tested at power-up or power-down.
Thus the state behaviour is modified by inserting an extra operation condition, namely a damped state following fault detection, and the elapsed period in the damped state before the shut-down state is reached.